@loopback/docs

# binding-resolution This example application is created to demonstrate LoopBack 4's context binding resolution and dependency injection within a context hierarchy. The application is verbosely instrumented to print out context/binding information to help developers understand how bindings are resolved and injected based on the current context and the binding scope. ## The application scenario ![binding-resolution](binding-resolution.png) ## The context chain When `GET http://127.0.0.1/ping` is served, the following context hierarchy is leveraged by various artifacts. - Application - RestServer - RequestContext (for HTTP requests) - InvocationContext (for interceptors) ## Binding scopes for key artifacts The artifacts below are bound to the context chain with certain scopes. - Spy middleware (Singleton or transient) - Spy interceptor (Singleton or transient) - Ping controller (Transient or singleton) - Logger service (Singleton) - Request Logger service (Transient) ## Binding resolution and injection 1. `new BindingDemoApplication()` creates the application context 2. The `RestApplication` registers `RestComponent` to the application. As a result, the `RestServer` is bound to the application context. The `RestServer` instance is the server context. 3. `app.boot()` discovers and loads the following bindings into the application context: - PingController - SpyInterceptor - LoggerService - RequestLoggerService 4. `app.middleware(SpyMiddlewareProvider)` binds `middleware.Spy` to the server context. 5. When an HTTP request `http://localhost:/3000/ping` is sent, the rest server accepts the request, creates a `RequestContext` and dispatches it to the middleware sequence. 6. The `Spy` middleware is invoked. It creates `LOGGER_SERVICE` in the request context as an alias to `RequestLoggerService`. 7. The `PingController.ping` is mapped for the route. A new instance of `PingController` is created within the request context. Its dependency of `LOGGER_SERVICE` is resolved and injected into the controller with an instance of `RequestLoggerService`. 8. The `Spy` global interceptor is invoked and it has access to the invocation context. 9. The `ping` method of `PingController` is invoked. Method parameter injection is performed to supply the `requestCtx` parameter value. 10. The `ping` method calls `Logger` service. ## Use ```sh DEBUG=loopback:example:binding-resolution npm start ``` To force binding scope to be `Transient`: ``` DEBUG=loopback:example:binding-resolution BINDING_SCOPE=transient npm start ``` Open http://localhost:3000/ping in your browser. The console will print similar information as below: ``` loopback:example:binding-resolution [middleware.Spy*] Owner context: RestServer-v2IUN1iNSM6NCqpa6D8vWw-1 +0ms loopback:example:binding-resolution [middleware.Spy*] Current context: RestServer-v2IUN1iNSM6NCqpa6D8vWw-1 +0ms loopback:example:binding-resolution [middleware.Spy*] Request context: RequestContext-v2IUN1iNSM6NCqpa6D8vWw-3 +0ms loopback:example:binding-resolution Request: GET /ping {...} +0ms loopback:example:binding-resolution [services.RequestLoggerService] Owner context: BindingDemoApplication-v2IUN1iNSM6NCqpa6D8vWw-0 +9ms loopback:example:binding-resolution [services.RequestLoggerService] Current context: RequestContext-v2IUN1iNSM6NCqpa6D8vWw-3 +0ms loopback:example:binding-resolution <<services.RequestLoggerService>> (GET /ping) [controllers.PingController] Owner context: BindingDemoApplication-v2IUN1iNSM6NCqpa6D8vWw-0 +0ms loopback:example:binding-resolution <<services.RequestLoggerService>> (GET /ping) [controllers.PingController] Current context: RequestContext-v2IUN1iNSM6NCqpa6D8vWw-3 +0ms loopback:example:binding-resolution [globalInterceptors.Spy*] Owner context: BindingDemoApplication-v2IUN1iNSM6NCqpa6D8vWw-0 +1ms loopback:example:binding-resolution [globalInterceptors.Spy*] Current context: BindingDemoApplication-v2IUN1iNSM6NCqpa6D8vWw-0 +0ms loopback:example:binding-resolution [globalInterceptors.Spy*] Invocation context: InterceptedInvocationContext-v2IUN1iNSM6NCqpa6D8vWw-4 +0ms loopback:example:binding-resolution <<services.RequestLoggerService>> (GET /ping) [controllers.PingController] Request context: RequestContext-v2IUN1iNSM6NCqpa6D8vWw-3 +1ms loopback:example:binding-resolution <<services.RequestLoggerService>> (GET /ping) Response {...} +1ms loopback:example:binding-resolution [middleware.Spy*] Request context: RequestContext-v2IUN1iNSM6NCqpa6D8vWw-5 +188ms loopback:example:binding-resolution Request: GET /favicon.ico {...} +0ms ``` ## Contributions - [Guidelines](https://github.com/loopbackio/loopback-next/blob/master/docs/CONTRIBUTING.md) - [Join the team](https://github.com/loopbackio/loopback-next/issues/110) ## Tests Run `npm test` from the root folder. ## Contributors See [all contributors](https://github.com/loopbackio/loopback-next/graphs/contributors). ## License MIT